Catalysis of CO₂ reactions by lung carbonic anhydrase

Steady state carbon dioxide excretion was studied in isolated bloodless lung preparations perfused with bicarbonate solutions. Addition of acetazolamide produced a prompt, significant decrease in the volume of excreted CO₂ under all conditions studied. Excreted CO₂ was derived from two sources: CO₂ dissolved in the perfusate and CO₂ produced by dehydration of bicarbonate in the pulmonary capillary. The relative quantity of these two soruces was determined by measurement of the simultaneous excretion of acetylene. Determination of the rate of CO₂ production permitted the calculation of capillary blood volume, mean capillary transit time, and the degree of catalysis of CO₂ reactions by carbonic anhydrase present in the lung. Contamination of perfusate with blood carbonic anhydrase was ruled out by measuring hemoglobin concentration and carbonic anhydrase activity in pulmonary venous drainage. Comparison of steady state CO₂ production during control conditions and carbonic anhydrase inhibition indicated that bicarbonate in plasma has access to sufficient lung carbonic anhydrase to catalyze the CO₂ hydration reaction by a factor of five.